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Tokyo: A device seen as key to explaining why a Boeing 787 Dreamliner jet made an emergency landing in Japan last week is burnt and unlikely to provide safety inspectors with data they need, said a person with knowledge of the ongoing investigation.

Circuit boards that control and monitor the performance of the plane's lithium-ion battery unit were charred and may be of little use to the teams investigating why the battery effectively melted, forcing safety investigators to scramble for possible clues from other components in the plane's electronics, said the person, who didn't want to be named as the probe is ongoing.

Painstaking reconstructionAviation authorities in Japan face a painstaking reconstruction that may take months before they can unravel what caused one of the batteries to overheat, triggering warnings in the plane's cockpit.

That inexpensive circuit boards may be keeping $10 billion worth of futuristic aircraft idle underscores how dependent the Boeing jet is on advanced electronics rather than more traditional, but less fuel-efficient, parts, experts said.

"The circuit board (system) is badly damaged. We'll see how much we can learn from examining it, but we'll also have to look at other recording devices on the aircraft to try and find out what happened," the person with direct knowledge of the investigation told Reuters.

Battery "brain" The circuit boards, known as the battery monitoring unit, are the "brains" of the battery, experts said. About the size of a laptop computer, the boards monitor functions of the lithium-ion battery's eight cells and feed this information to the charger. That makes the boards responsible for preventing a battery from overcharging. One key question for safety investigators is how the battery's eight individual cells became volatile even though the overall voltage to the battery was steady and didn't exceed the 32-volt capacity, officials have said. That data is not recorded in the Dreamliner's "black box" flight-data recorder.